Engineering & Mining Journal

SEP 2017

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TAILINGS MANAGEMENT 54 E&MJ • SEPTEMBER 2017 www.e-mj.com ecutive, who said he loved that filter, but commented it was too small. Wisdom re- minded him it was twice the size of any filter press currently on the market. He said, "Yeah, I know, but we need some- thing bigger and better." At that point, the engineers at FLSmidth realized they needed a new approach. As point of reference, Wisdom cited a mining operation with some clays in the deposit. To process 150,000 mt/d of filtered tailings, they would have need- ed 72 2-m x 2-m filter presses. "If you tell a mine they need 72 of anything, they are going to get squeamish," Wis- dom said. "You could cut that figure in half with 36 Colossal Filters, but that's still a big proposition." The pressure filtration process is a batch operation. So, they started to look at reducing the steps of the batch pro- cess. That led to reducing the air blow in a filter for fast filtering with a wetter filter cake. "We could reduce that number by half again," Wisdom said. "So we went from 72 filters to six or eight and that's much more reasonable for them as far as operations and maintenance." Reducing the air blow increases moisture, which yields less geotechnical strength for the filtered tailings. "During the last six years or so, most mines were looking for filter cake with 13% to 16% moisture on a metallurgical basis," Wis- dom said. "To get to that level requires quite a bit of air blow. By increasing that figure to 18% to 20%, the filtration time can be reduced significantly, meaning more batches per hour and higher unit capacities. The problem is dealing with that wetter filter cake." Editor's note: Metallurgical engineers and geotechnical engineers use a per- centage figure to describe moisture dif- ferently. A metallurgical engineer uses a Considerations for Modeling Tailings By Joe Kraft, mine planning specialist, Minemax With strategic mine planning, looking at large horizons of time and the varying interdependencies of the components that make up the operation, tailings disposal is an area that must be addressed. What do planners do when they are asked to work out a sequenced delivery of pit material to help construct new tailings dams? What if that material has to be of a specific composition and it's not readily available in the mining faces? What if multi- ple cells of this facility will be constructed over time? And final- ly, what if the mill can't come online until this tailings facility is ready? That means that any delay in the delivery of material will directly affect revenue generation and the bottom line for the business. A case like this has a complex situation that demands smart, integrated planning of multiple downstream components over the life of the project. This would be tough to accomplish with a spreadsheet or a basic heuristic scheduling package, however, this is the type of planning Minemax Scheduler was developed for. Defining the Problem Consider a typical situation of an open pit with multiple phases feeding a single mill that produces gold and tailings. In this situation, planners need to solve for cut-off grade, therefore, a block from the mine can go anywhere that gives the greatest net present value (NPV), subject to the conditions and constraints in each period. For every block that's processed, there's a calculat- ed mass of tailings produced with a given volume, which reports to a tailings storage facility (TSF) with a fixed capacity. That TSF requires a designed embankment structure (dam) that must be constructed of a certain kind of waste material that is only available in certain parts of the pit. The model must incorporate the requirement to send suitable material to build this structure before a single block can be processed in the mill. Model Definition To incorporate destination requirements as well as timing decisions, Minemax Scheduler models mill (Mill_Tonnes) and waste destinations (Waste_Tonnes) as alternative options using a decision tree diagram. During the optimization, the econom- ic value and impact of multiple parallel constraints determine whether material is sent to the mill for processing, or is con- sidered raw waste. If material is sent to the mill for processing, processed waste (Tailings) is produced and placed in the TSF (vol-Tailings) and is costed accordingly. Raw waste (Waste_Tonnes) that is not going in the mill is modeled using an alternative decision. It can be either sent to the waste dump (vol-Waste Dump) or the tailings dam (vol-Tail- ings Dam). There is an obvious haulage difference between the two options, and so we model this using the truck hours (TH) processes Waste_TH_Tailings and Waste_TH_Dump. The TSF location (vol_Tailings) is unavailable for storing tail- ings until the tailings dam is finished. Minemax Scheduler han- A decision tree helps visualize the modeling process.

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